For all the talk about there being a lot of dinosaurs – and I’m speaking here of fossil non-bird dinosaurs from the Mesozoic Era – and about this being a golden age where discovery rates are at an all-time high, the fact is that there really aren’t that many dinosaurs at all.
Caption: these years, just about every year is a dinosaur year. Here are just a few of the many Mesozoic dinosaurs published as new in 2019. Yeah yeah, don’t be a smartass - the people are there as scale bars.
Yes, you read that right. There are whole swathes of the Mesozoic – sections of the geological record which correspond to chunks of time lasting for several millions of years – where, worldwide, we know of just a few dinosaur species, sometimes none at all. This means that – even in this golden age of discovery, where around 50 new non-bird dinosaur species are named every year – there are still huge numbers of dinosaurs awaiting discovery, plus of course there are surely some great many that will never be discovered at all seeing as their fossils have been entirely destroyed.
Caption: a Morrison Formation exposure in Utah. These rocks - or, more specifically, the environments represented by them - are the setting for the article here (and its several subsequent parts). Image: Michael Overton, CC BY-SA 2.5 (original here).
There are, however, a few segments of Mesozoic history and a few regions of the globe where things are not all that bad, and where we do have what looks like a good sampling of dinosaur diversity. Take the Upper Jurassic Morrison Formation of the continental western interior of the USA. The number of sauropod species and genera reported from the Morrison is really impressive, and includes as many as 30 currently recognised species. Waitaminute… 30 species of gigantic megaherbivores, all living in the same geographic region at the same time? Surely this can’t be so, and surely any efforts to recognise new taxa, or resurrect old ones out of synonymy, can only be wrong? Surely there are too many damn dinosaurs.
Caption: we’ll be talking about both familiar and obscure Morrison Formation sauropods here. This mural image by Mark Witton - revamped several times over the years, as discussed by Mark here - depicts Diplodocus longus. Image: (c) Mark Witton.
J’accuse, with a diversion on the OMH. This contention – too many damn dinosaurs, or TMDD from hereon – has only been put forward on a small number of occasions. Admittedly, most of the literature that can be interpreted as supporting this view doesn’t push the TMDD view for the ecological reasons mentioned above. It is instead driven by an effort to push the ontogenetic morphing hypothesis (OMH), the model which proposes that dinosaur specimens mostly accepted as distinct species or genera are in fact growth stages of another one. While there surely must be (or are) cases where the OMH does explain the variation we see in fossil dinosaurs, I can’t help but be mostly sceptical of it.
Caption: poster-children of the ontogenetic morphing hypothesis. Nanotyrannus, Dracorex and Torosaurus. Some Mesozoic dinosaurs surely did undergo surprising change across ontogeny. It doesn’t mean they all did. Images: James St. John, CC BY 2.0 (original here); Daniel Hendricks, CC BY-SA 2.0 (original here); Kabacchi, CC BY 2.0 (original here).
Why? Firstly, because some of the researchers concerned state often that they dislike phylogenetics, and it’s phylogenetics which shows how different specimens belong to different lineages within a group (rather than being segments of the same lineage or even members of the same species). Secondly, because I’m concerned that the enormous popularity of the ‘Toroceratops’ idea*, the death of Stygimoloch and Dracorex** and the death of Nanotyrannus*** have driven an attempt to synonymise other dinosaurs, more because it seems like an avant-garde line of research than an obviously worthy one. And thirdly…. because it just looks wrong. I am not buying that, say, Haplocanthosaurus magically morphs into Diplodocus during ontogeny (for more on that specific case see the series of articles at SV-POW!).
* which proposes that Torosaurus is the same taxon as Triceratops. I think it’s wrong.
** which proposes that Stygimoloch and Dracorex are growth phases of Pachycephalosaurus. I think it’s probably right.
*** which proposes that Nanotyrannus is a juvenile of Tyrannosaurus. I think it’s almost certainly right.
Caption: composite skeletal reconstruction of Haplocanthosaurus, a Morrison Formation sauropod that may be closely related to - or an early-diverging member of - Diplodocoidea. Image: IJReid, CC BY 4.0 (original here).
With this in mind, who’s been pushing the TMDD claim? Among Morrison Formation sauropods, Woodruff & Fowler (2012) proposed that Suuwassea, Haplocanthosaurus and Barosaurus might be growth stages of the diplodocids Diplodocus and Apatosaurus, a surprising contention to say the least. They continued to push this idea for Suuwassea (Woodruff et al. 2017) even after Wedel & Taylor (2013) strongly refuted it on the basis of abundant anatomical information and Hedrick et al. (2014) tested it via histology and found it to be contradictory to data. And in their evaluation of Amphicoelias fragillimus – since renamed Maraapunisaurus (Carpenter 2018) – Woodruff & Foster (2014) implied (but did not outright state) that this supposedly gargantuan Morrison sauropod couldn’t have existed for ecological reasons.
Caption: views on Maraapunisaurus (formerly Amphicoelias fragillimus) have varied quiet a bit since I first wrote about it in 2006 (see links below). Most recently, Carpenter (2018) has argued that it was most likely a rebbachisaurid, not a diplodocid, and with a total length of perhaps 30 m. The reconstruction here depicts it as a rebbachisaurid. Note the typo in ‘fragillimus’. Image: Slate Weasel, CC0 (original here).
More recently, Don Prothero – in his 2019 book The Story of the Dinosaurs in 25 Discoveries (Prothero 2019) – has also cried TMDD, this time because he has misgivings about Tschopp et al.’s (2015) proposal that Brontosaurus should be regarded as distinct from Apatosaurus. In that book (the relevant section of which mostly repeats arguments initially shared in a 2015 article published at the Columbia University Press blog), Don stated: “I’m concerned that all these names for similar-sized sauropods living in the same region are not justified. There is no clear evidence that they had the room or the diversity of food sources or habitats to allow so many species to live close together, all with monstrous appetites as befits their huge size” (p. 111). He also stated that big sauropods didn’t have access to the sorts of resources that would have allowed persistence at high diversity, and furthermore pointed to work (including his own) whereby supposed species and genera of rhino and brontothere were synonymised after being shown to be oversplit (Prothero 2019, pp. 110-111).
Caption: Prothero (2019). I was a reviewer.
Ok, there’s a lot to unpack here, and in the several articles which follow this one I’m going to go through most of the things I think are relevant to the TMDD claim. We’ll start by looking at Don Prothero’s argument that the taxonomic history of fossil mammals provides support for the TMDD contention, then at a discussion Prothero provided on giraffes and their relevance. We’ll then change tack and look at Morrison Formation geology to see what it tells us about sauropod diversity and distribution. We’ll continue by looking at some specific aspects of sauropod biology, ecology, distribution and diet relevant to the TMDD claim, and finally at the application of what we might term appropriate expertise. If – say – you’re reading part 2 in the series and are wondering why I haven’t mentioned subject x or y, stay tuned for the later parts, as they should get covered eventually. I have these articles lined up and ready to go, by the way, so should be publishing them in fairly quick succession – no long waits of weeks or months between the sections.
Caption: it should be obvious that I don’t have enough scenes depicting Morrison Formation dinosaurs, since I’ve had to use this one about 50 times by now. The sauropods are Diplodocus… again. Image: Darren Naish.
Finally, I should add that I don’t wish to sound rude, disrespect or mean to the researchers mentioned throughout this series of articles. I’ve worked with both Cary Woodruff and Don Prothero and regard them as friends, but I do disagree with them on this issue.
Let’s get to it… come back soon for Part 2 in this series.
For previous TetZoo articles on sauropods and related issues (linking where possible to wayback machine versions), see…
Biggest…. sauropod…. ever (part…. I), January 2007
Biggest sauropod ever (part…. II), January 2007
The hands of sauropods: horseshoes, spiky columns, stumps and banana shapes, October 2008
Sauropod dinosaurs held their necks in high, raised postures, May 2009
Paul Brinkman’s The Second Jurassic Dinosaur Rush, March 2011
The sauropod viviparity meme, May 2011
Necks for sex? No thank you, we’re sauropod dinosaurs, May 2011
The Second International Workshop on the Biology of Sauropod Dinosaurs (part I), December 2011
The Second International Workshop on the Biology of Sauropod Dinosaurs (part II), January 2012
Greg Paul’s Dinosaurs: A Field Guide, February 2012
Junk in the trunk: why sauropod dinosaurs did not possess trunks (redux, 2012), November 2012
That Brontosaurus Thing, April 2015
10 Long, Happy Years of Xenoposeidon, November 2017
The Life Appearance of Sauropod Dinosaurs, January 2019
Refs - -
Woodruff, D. C. & Foster, J. R. 2014. The fragile legacy of Amphicoelias fragillimus (Dinosauria: Sauropoda; Morrison Formation-latest Jurassic). Volumina Jurassica 12, 211-220.
Woodruff, D. C. & Fowler, D. W. 2012. Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character. Journal of Morphology 273, 754-764.
